1. Field of the Invention
The invention relates to system and method for detecting road lane boundaries, and more particularly to such system and method for detecting road lane boundaries, based on image data representing a captured image of a road ahead of the vehicle.
2. Description of the Related Art
A road lane boundary detection system for a vehicle is known which is operable to detect road lane boundaries, such as white lines, which are drawn on a road and define a lane in which the vehicle runs. This system is configured to detect road lane boundaries by processing an image (image data) captured by a camera installed on the own vehicle for taking pictures of a region ahead of the vehicle. For example, the system extracts an image of a certain specific size to be compared, from the captured image, and determines whether the extracted image to be compared matches any of registered images relating to particular objects, using image processing, such as digitization, edge detection and filtering, of the image. If it is determined that the extracted image matches a registered image relating to a particular object, it is recognized that the particular object is present in the captured image. In this manner, white lines, or the like, are detected. The road lane boundary detection system of this type is required to detect white lines, or the like, on the road ahead of the vehicle, from moment to moment at very short time intervals of 1/10 sec. to 1/100 sec. However, an enormous amount of computations are needed to carry out a detecting process for detecting white lines, or the like, with respect to all of the pixels of the captured image. Accordingly, various approximations or simplifications are performed in each step of the process leading to detection of white lines, or the like.
For example, an image recognition system (as disclosed in, for example, Japanese Patent Application Publication No. 2007-235414 (JP-A-2007-235414)) is configured to limit a range to be searched for a certain object or objects, in an image captured from a mobile unit, so as to reduce the load on the system for image recognition processing. With this technology, a range within which imaging is possible is divided into a range delimited by a boundary to which the vehicle is able to reach after a lapse of a specified time, and a range beyond the boundary, into which the vehicle is not able to reach within the specified time. Then, the system limits a range on which the image recognition process is performed, to the range that can be reached by the vehicle, so as to reduce the processing load on the system for processing, such as extraction of an image to be compared with the registered image(s), and comparison between the image to be compared and the registered image(s).
Also, a system (as disclosed in, for example, Japanese Patent Application Publication No. 2007-72512 (JP-A-2007-72512)) is configured to set a close region that is relatively close to the vehicle and a remote region that is farther than the close region, in an image captured from a mobile unit, for use in detection of road lane boundaries, such as solid lines or broken lines. With this technology, when the system reads image data, it divides a region to be processed in the image, into the close region that is relatively close to the vehicle, and the remote region that is farther than the close region, and set these regions for use in subsequent processing. Also, the system has a solid line detection mode for detecting solid lines, and an intermittent line detection mode for detecting broken lines. In the solid line detection mode, the system sets twenty detection lines parallel to the X axis so as to equally divide the distance on the Y axis in the region into twenty sections, with respect to each of the close region and the remote region. In the intermittent line detection mode, on the other hand, the system sets forty detection lines parallel to the X axis so as to equally divide the distance on the Y axis in the region into forty sections, with respect to the close region, for example. Here, the detection lines set on the image provide equally spaced lines in real space. FIG. 16 is a view showing the corresponding positions of the detection lines in the real space, which indicates detection lines on the road as viewed from above. Since the detection lines are arranged so as to be spaced at equal intervals in the real space, as shown in FIG. 16, the detection lines provide unequally spaced lines on the image, as shown in FIG. 17. Once the system sets the detection lines in this manner, it differentiates the brightness (density) of pixel in the X-axis direction, with respect to each pixel on each detection line, and extracts or picks up coordinates of points at which the absolute value of the differential exceeds a threshold value A, as feature points, based on which road lane boundaries are detected.
However, the systems as disclosed in the above-identified patent publications suffer from some problems as follows. First, so-called Botts dots (dots each having a diameter of about 10 cm) are known as one type of road lane boundaries or markings provided on the road in place of white lines. Unlike the white lines, the Botts dots are intermittently arranged or spaced at given intervals on the road. On the road on which each road lane boundary consists of these intermittent markings, it would be easy to assume or recognize a single road lane boundary from these intermittent markings via human eyes. However, in the case where a system, like the systems as described above, mechanically recognizes road lane boundaries in the form of imaginary or substantial lines, through image processing on an image that is captured by a camera and that includes the intermittent markings, the road lane boundaries may not be appropriately detected. Even if the region to be processed on the captured image is limited as in the system of JP-A-2007-235414, and a detecting process is performed using spaced detection lines by which the distance on the Y axis in the region is equally divided into twenty or forty sections, as in the system of JP-A-2007-72512, feature points, or the like, associated with Botts dots that are present between the detection lines cannot be acquired, due to the fact that the Botts dots are intermittently arranged, and accurate detection cannot be achieved.
It may be proposed to subject all lines (all pixels) of the captured image to the processing for detection, without extracting or selecting detection lines from all lines. In this case, however, an enormous amount of computations are needed, and the processing load increases, thus making it difficult to quickly acquire necessary information. It may also be proposed to divide the region to be processed in the image, into a close region that is relatively close to the vehicle and a remote region that is farther than the close region, and limit the object to be processed to the close region so that all lines of the close region are searched for detection, thereby to reduce the processing load. Although this method is useful in a detecting process performed, for example, when the vehicle starts running, or for the purpose of “finding” Botts dots, etc., it is necessary to continue searching for Botts dots on the road (i.e., tracking Botts dots) after the Botts dots are initially found, so as to recognize the Botts dots as road lane boundaries. In this case, since the object to be processed is limited to the close region, the detecting process is only performed on, for example, a region up to 10 m ahead of the vehicle, resulting in a problem with the accuracy with which Botts dots are detected during running of the vehicle, for example.